In a known energy management system used to record consumption of electricity at consumers' premises, electronic registers are used to record the amount of electricity consumed during a number of different tariff rate periods per day. Thus there may be a high tariff rate register, a medium tariff rate register and a low tariff rate register and a time of day clock determines whether consumption occurs during a peak period, a normal period or a low cost period so the meter registrations are recorded in the appropriate register. It is thus necessary for the time of day clocks at each consumer's meter to be accurately locked into the time of day within a set tolerance of e.g. one minute.
In a system where a central computer can monitor the meters at each consumer's premises over the consumer's line, a time signal may be transmitted from the central computer at regular intervals, e.g. once a day. However, the transmission process has an inherent random delay while accessing the subscribers' energy management terminals (EMT). This delay may be of the order of 30 seconds between when the central computer transmits the time signal and when it is received by the EMTs.
There will thus usually be a difference between the time registered by the EMT's time of day clock and the clock synchronizing signal received from the central computer due to drift in the EMT clock and due to the random transmission delay. Both the drift and the delay are unknown parameters but a maximum drift will usually be specied in the tolerances for the EMT. This may be, e.g. 0.04 s per 15 minutes (40 per day).
A further complication may be added if the requirements of the electricity supply authority require that the time intervals for the consumption periods be within a given tolerance. In the US the electricity metering standard ANSI C12.13 requires an accuracy of 15 minutes.+-.0.9 seconds.
This latter requirement means that the EMT clock cannot automatically be reset in synchronism with the received clock synchronization signal because such a time step may exceed the permissible tolerance.
It is an object of the present invention to provide a method of and an apparatus for adjusting the pulse rate of a local clock to obtain a more accurate time signal therefrom in the absence of an accurate clock synchronization signal.
It is a further object of the present invention to provide a method of and an apparatus for adjusting the pulse rate of a local clock to obtain a more accurate time signal therefrom when using a clock synchronization signal for synchronizing said local clock transmitted from a remote station via a communication link which introduces random delays to the transmission of the synchronization signal.